The present invention relates to a printing apparatus of the electrophotographic type for developing an image using colored particles like toner, such as a printer, a facsimile, a copier and the like; and, more particularly, the invention relates to an electrophotographic printing apparatus using a developing method in a developing process for forming a toner image on a surface of a printing medium.
A conventional developing method and developing unit will be described below. A printing apparatus of the electrophotographic type uses a developing process for forming a colored particle image on a surface of a medium and a fixing process for fixing the colored particle image onto the medium. Powder called toner for electrophotography is used for the colored particles.
In the developing process, initially the colored toner particles are charged using a developing agent. The developing agent is a mixed powder of toner and carrier beads of magnetic particles. The developing agent is contained in a developing unit and mixed therein. The toner is charged by friction with the beads generated at that time. In image forming in the developing process, a method called bias development is commonly used. A photosensitive body is first charged over the whole surface thereof, and then the charge is selectively discharged by irradiating light thereon. Therein, an electric potential pattern (electrostatic latent image) is formed on the surface of the photosensitive body consisting of charged zones and discharged zones.
In the bias development, a bias voltage is applied to a magnet roller called a developing roller, which transfers developing agent up to a position opposite a developing position where an electrostatic latent image is carried on the surface of the photosensitive body. By doing so, the charged toner particles are separated from the developing agent on the surface of the developing roller and are transferred to the surface of the photosensitive roller by the action of an electric field generated between an electric potential of the latent image formed on the surface of the photosensitive body and the developing roller. Thus, a toner image is formed.
The difference between the bias voltage of the developing roller and the electric potential of a latent electrostatic image formed on the photosensitive body is referred to as a developing electric potential difference. There is no need to say that, when the developing electric potential difference is large, the formed electric field (called developing electric field) becomes strong and, accordingly, the developing performance is improved. A method involving the narrowing of the distance between the developing roller and the photosensitive body, or a method which calls for decreasing the electric resistivity of the developing agent also has the same effect of strengthening the developing electric field and improving the developing performance.
A modified version of the electrophotography bias developing described above is proposed in, for example, Japanese Patent application Laid-Open No.48-37148. In this developing method, development using two kinds of toner is carried out with one charging process and one irradiating process (exposing process) by providing an intermediate electric potential zone, by dividing an electric potential between a charged zone and a discharged zone on a photosensitive body, and by developing the discharged zone with a first toner using a first developing unit for performing reverse development of the discharged zones, and then developing the charged zone using a second toner with a second developing unit for performing normal development of the charged zones.
In this developing method, the toner is not attached in the intermediate electric potential zone on the photosensitive body having a voltage value between the bias voltage value of the first reverse developing unit and the bias voltage value of the second normal developing unit, and, in this way, a background portion is formed as an image. Therefore, it is possible to form an image composed of a background portion, a first image portion and a second image portion using two kinds of toner. In this specification, this developing method will be referred to as a method of the electric potential dividing development type. In general, the two kinds of toner used are classified by color, and are used to obtain an image composed of two colors. Further, in principle, the electric potential dividing development method can be performed by setting the first development to a normal developing and the second development to a reverse developing.
In the bias development, when an electrostatic latent image is formed on a photosensitive body, an electric field emphasizing the development is produced at an end portion of the latent image, and at the same time an electric field having a polarity opposite to the polarity of the latent image (a reverse electric field) is generated at the periphery of the latent image. In the case of bias development, which uses one kind of toner and employs the electric potential dividing developing method, this reverse electric field is not a problem in forming an image.
However, in the electric potential dividing development process in which positive and negative electrostatic latent images are formed on the photosensitive body and two kinds of toner (hereinafter, taking red toner and black toner as an example), charged in polarities respectively opposite to the polarities of the images, are developed, there occurs a fringe development in which red toner attaches around a black image and black toner attaches around a red image due to the attraction of the reverse electric fields. The fringe development represents an erroneous printing in that toner is attached at an unexpected position.
Characteristics of the fringe development will be described below with reference to FIGS. 10A to 10C. FIG. 10A is a diagram showing forces acting on toner on a surface of a photosensitive body. The toner attached to a leading end portion of a spike formed by the developing agent is moved toward the photosensitive body from the developing roller by an electric field force qEz, which is expressed by the product of an electric field Ez in an opposite direction due to an edge effect in the surrounding area of an image portion and a charge amount q of the toner.
The peripheral speed of the surface of the developing roller is generally set to be faster than the peripheral speed of the surface of the photosensitive body in order to improve the developing performance. In the illustrated example, the photosensitive body and the developing roller are rotated in the same direction, and a friction force FR by the spikes of the developing agent acts on the toner in the positive Y-direction caused by the difference between the peripheral speeds of the surfaces of the photosensitive body and the developing roller.
In addition to the electric field force qEz and the friction force FR, a force qEy in a direction along the surface of the photosensitive body at a peripheral portion of the image portion caused by a difference between the electric potential of the image portion and the electric potential of a white portion at the periphery thereof acts on the toner. If the image is at an electric potential of the discharged zone, the electric field Ey is generated so as to be directed outward from the peripheral portion of the image. If the image is at an electric potential of the charged zone, the electric field Ey is generated so as to be directed inward from the peripheral portion of the image.
The toner development by the electric field in the opposite direction due to the edge effect at the periphery of the discharged electric potential zone has a certain amount of positive charge, and the toner developing by the electric field in the opposite direction due to the edge effect at the periphery of the charged electric potential zone has a certain amount of negative charge. Therefore, the electric field force qEy acts on the toner in a direction outward from the peripheral portion of the image in both cases. In the present specification, the electric field in the opposite direction is referred to as a reverse electric field.
FIG. 10B is a view showing forces in the direction along the surface of the photosensitive body which act on the toner at the front end and the rear end of the image relative to the rotating direction of the developing roller. The directional relationship between the friction force FR and the electric field force qEz is different between the front end and the rear end of the image. At the front end of the image, the friction force FR and the electric field force qEz act in the same direction, and the toner developing the fringe is scraped off from the edge portion of the image. On the other hand, at the front end of the image, the friction force FR and the electric field force qEz act in directions opposite to each other, and the toner developing the fringe is retained at the edge portion of the image.
FIG. 10C is a view showing a feature of the fringes which occur at the front end side and the rear end of the image, in respect to the rotating direction of the developing roller, which are produced by the above-mentioned forces acting on the toner on the surface of the photosensitive body. A stronger fringe development appears at the rear end of the image than at the front end thereof. If the intensity of the fringes is suppressed to a certain degree by the resistivity of the developing agent and the developing bias using the difference in the characteristics of fringe intensity between the front end and the rear end of the image, what remains is only the fringe at the rear end of the image, which has the stronger intensity.
In order to eliminate erroneous printing caused by fringe development, it can be considered that an electric potential dividing developing method without fringe development can be realized by introducing exposure control (hereinafter, referred to as fringe control) in which an expected position of occurrence of fringe development is predicted and an auxiliary exposure is applied to the expected position to suppress the occurrence of a reverse electric field.
When the above-mentioned method is employed, it is required to supply a different exposure amount of light to each of the fringe around a red image and the fringe around a black image as an auxiliary exposure, and drivers are respectively required corresponding to the set exposure amounts when an analog light emitting element driver cannot be used. Further, when a fringe control means is used, there is a problem in that a reverse electric field generated by the auxiliary exposure light causes additional fringe-like erroneous printing. In this specification, the fringe-like erroneous printing caused by auxiliary exposure of light is referred to as a fringe caused by auxiliary exposure.
The conventional electric potential dividing development method described above fails to give consideration to fringe development, and fails to give sufficient consideration to a condition and method for preventing occurrence of a fringe caused by auxiliary exposure. Therefore, the conventional electric potential dividing development method has a problem in that a fringe caused by auxiliary exposure is substantially generated under a condition of a large peripheral effect of an electric field, such as when employing a high resistivity developing agent.
Further, in a case of employing fringe control, the size of the control circuit is not taken into consideration. When an analog light emitting element driver cannot be used in order to supply a different exposure amount of light to each of a fringe around a red image and a fringe around a black image in providing the auxiliary exposure, there is a problem in that individual drivers are required corresponding to each of the set exposure amounts, respectively.